Compositions that repel insects, particularly insect pests such as mosquitoes, flies and fleas, are widely used to prevent animals from being bitten by such insects. The importance of preventing biting is essentially two-fold. Firstly, there are a number of insects which are capable of infecting animals with disease causing parasites, an example being the transmission of Plasmodium by mosquitoes to cause malaria. Secondly, in many cases whether disease is transmitted or not, the bite can be extremely irritating.
Of course, man is one animal that suffers from biting insects. Man also suffers irritation from non-biting, annoying insects such as domestic flies and from insects that transmit disease by contact.
The prior art discloses many compositions having repellency to some or a number of insect species. Of these compositions, the most widely used includes the active ingredient N,N-diethyl-m-toluamide (DEET).
Generally, compositions based on DEET are in the form of lotions or aerosols to be applied or sprayed onto the exposed skin. Such compositions may include a variety of other ingredients such as film forming agents to enhance substantivity to the skin and potentiators such as those disclosed in WO89/06904-A.
Although DEET containing compositions are relatively effective particularly in repelling domestic flies and mosquitoes, concerns have been expressed about its toxicity to humans.
Other compositions that are known repellents include various extracts of naturally occurring compounds obtained from eucalyptus species. One series of naturally occurring compounds obtained from Eucalyptus citriodora are the p-menthane-3,8-diols. These compounds are known to exert a repellent effect on insect pests such as mosquitoes and fleas.
Thus in Nishimura et al. Agric. Biol. Chem. 46(1) 319 (1982), the authors disclose the isolation of cis and trans p-menthane-3,8-diols in the form of racemic mixtures from Eucalyptus citriodora. In addition (+) and (-) cis and (+) and (-) trans isomers were synthesized. All compounds were tested for their effectiveness as plant growth regulators but no mention is made of any insect repellent properties.
In an earlier publication, J. Am. Chem. Soc. 75, 2367 (1953), Zimmerman and English report the synthesis of the cis and trans p-menthane-3,8-diols, designated Va and Vb respectively, through the acid cyclization of citronellal. Again no mention is made of any insect repellent properties.
In JP60-199804-A (Nippon Kayaku KK), it is disclosed that the cis and trans p-menthane-3,8-diols are effective insect repellents, as are a number of analogues. This patent application teaches that the p-menthane-3,8-diols may be prepared using the method disclosed in J. Am. Chem. Soc. 75, 2367 (1953). Using (+)-citronellal as the starting material and a reaction time of 27 hours at room temperature cis:trans p-menthane-3,8-diols in a ratio of 5:2 are obtained. These compounds are obtained in pure form following chromatographic separation and recrystallization. There is no mention of the use of the so-purified p-menthane-3,8-diols with any other insect repellent.
EP 367140-A (Takiron Co. Ltd) also relates to the use of p-menthane-3,8-diols as insect repellents. This application teaches slow-release compositions wherein the diols are included in compositions with an ethylene/vinyl acetate copolymer.
Although the p-menthane-3,8-diols are effective insect repellents, even the most efficient routes of synthesis currently known results in products that are so costly that repellent compositions including these menthanediols are too expensive compared with known alternatives.
However, in view of the effectiveness of the menthanediols, considerable effort has been put into developing more cost efficient routes of synthesis.
To that end the present applicants in investigating more efficient routes of synthesis pursued a synthetic route in which the starting material, citronellal, was subjected to acid catalysed cyclization (Prins reaction). This reaction causes the cyclization of the citronellal to produce an impure mixture of 40-45% cis and trans p-menthane-3,8-diols and is essentially the method disclosed in J. Am. Chem. Soc. 75, 2367 (1953). This impure mixture also includes unreacted citronellal, compounds having a boiling point about the same as or lower than citronellal and high boiling point compounds.
As it has been found that the cis isomer is more active than the trans isomer, the cis isomer may be isolated from the above mixture and used in its pure form. For the purposes of comparison, the cost of the impure mixture is significantly lower than that of cis isomer of 99% purity.